1. Field of the Invention
The present invention relates to a method for manufacturing a semiconductor device formed on an insulating substrate (such as glass) or on an insulating film formed on a substrate of various kinds. The semiconductor device includes, for example, thin film transistors (TFT), thin film integrated circuits constructed of TFTs, and thin film integrated circuits for liquid crystal displays of active matrix type. More particularly, the present invention relates to a method for heat treatment to obtain a gate insulating film having good characteristics.
The present invention is applicable to an insulated gate field effect transistor of a top gate type having the gate electrode above the channel forming region in which at least part of the channel forming region under the gate insulating film is amorphous or polycrystalline (or non-single crystalline) semiconductor, or a single crystalline semiconductor.
2. Description of the Related Art
A recent development in the semiconductor industry is a semiconductor device having thin film transistors (TFTs) on an insulating substrate such as glass. Such a semiconductor device is used for an active type liquid crystal display and image sensors in which TFTs drive picture elements.
The TFTs used for these devices are usually made of silicon semiconductor thin film, which is amorphous or polycrystalline. The amorphous or polycrystalline silicon semiconductor is non-single crystalline semiconductor and has recombination centers due to dangling bonds or the like. Recombination centers are present not only inside the semiconductor but also at the interface between the semiconductor and the gate insulating film.
Recombination centers greatly affect the characteristics of TFT. That is, an insulating gate type field effect transistor of non-single crystalline silicon is inferior in mobility and subthreshold characteristics (S value) to an insulated gate type field effect transistor of single crystalline silicon. Also, the former suffers a large leakage current through the gate electrode. This leads to a marked decrease in ON current (due to deterioration and change with time) after use for a long period of time.